Effects of disturbance and seed addition on plant community assembly and ecosystem functions

Previous project phase (2014 - 2017)

Scientific investigators:

Prof. Dr. Norbert Hölzel

PD Dr. Till Kleinebecker

Dr. habil. Ute Hamer

Dr. Valentin Klaus

(Uni Münster)

Background

Experiments controlling plant diversity have often demonstrated positive relations between the number of plant species and ecosystems functions such as productivity, biomass quality, nutrient cycling, temporal stability and resistance against perturbations and invasions. Yet, there is an ongoing debate how these findings from rather artificial and strongly controlled experiments scale up to real-world ecosystems. In our project, we will specifically address the effects of land-use intensity, pre-disturbance plant diversity, soil seed banks and assisted migration though seed addition on ecosystem resilience after heavy disturbance. A particular focus will be given to the impact of persistent soil banks as a hidden part of local diversity and introduced seeds on community reassembly and the recovery of ecosystem functioning.

Hypotheses

In the combined full-factorial disturbance-seed addition experiment SADE we will specifically ask the question how assisted migration by sowing and heavy soil disturbance affect community reassembly and essential ecosystem functions along a real world land-use gradient. A particular emphasis will be given to the resilience of community structure and productivity-related ecosystem functioning. In a complementary mesocosm experiment with grassland sods we will test if and how plant diversity modifies effects of experimental fertilizing and drought on ecosystem functions such as nutrient cycling. Specifically we will address the following hypotheses:

The seed and bud bank of the grasslands has a crucial importance for community reassembly after a drastic disturbance event.

At disturbed sites, subordinate and transient species emerging from the seed and bud bank will (temporarily) play a prominent role for ecosystem functioning.

In species-rich grasslands, diversity and ecosystem function will recover more quickly after disturbance to the original level due to higher propagule pressure from seed rain and the permanent soil seed bank.

Species enrichment by seed addition will enhance ecosystem functions such as biomass production, fodder quality, drought resistance and completeness of nutrient uptake, at least in previously species-poor grasslands.

Plant diversity will positively affect nutrient retention due to reduced losses to the groundwater or deeper soil layers and buffers drought effects on nutrient cycling and productivity.

Methods

Our project will analyse seed and bud banks on all 75 plots in the three study regions where the SADE experiment is implemented to estimate the importance of (hidden) plant diversity for community reassembly after a disturbance event. As major response variables for recovery of ecosystem functioning we will analyse productivity and nutrient stocks in aboveground biomass, including 15N and 13C isotopic signals as proxies for nitrogen uptake and drought resistance. Furthermore, mineralization rates of N and P in the soil using resin bags across all plots will help to assess nutrient availability and excess in relation to the diversity and land-use gradient. Interactive effects of plant diversity, drought stress and fertilization on nutrient cycling and productivity will be specifically addressed in a controlled mesocosm experiment aiming at the disentangling of confounding effects and the identification of key mechanisms.